In the kraft pulping process, wood chips or other cellulosic fibrous material are contacted with a pulping liquor comprising sodium hydroxide and sodium sulfide (white liquor). The resulting wood pulp is separated from spent pulping liquor (black liquor) and further processed. The spent pulping liquor is processed through a chemical recovery and regeneration cycle to form fresh pulping liquor.
Such a chemical recovery and regeneration cycle includes burning concentrated black liquor under reducing conditions in a recovery furnace, dissolving the inorganic combustion residue (smelt), mostly in the form of sodium sulfide and sodium carbonate, in water to form green liquor, and causticizing the green liquor with slaked lime (Ca(OH).sub.2), in a recausticizing plant to produce NaOH, in accordance with the equation: EQU Na.sub.2 CO.sub.3 (aq)+Ca(OH).sub.2 (s).fwdarw.2NaOH(aq)+CaCO.sub.3 (s)
The precipitated calcium carbonate from the causticization reaction is separated from the regenerated white liquor and washed. The resulting lime mud is calcined to form lime, which is slaked with weakwash (water) to produce Ca(OH).sub.2 for reuse. In a typical kraft pulp mill operation, approximately 0.26 tons of lime are required per ton of unbleached pulp produced.
Calcination of lime mud usually is carried out in an inclined rotary kiln, although flash calciners may be used. A lime kiln typically is dimensioned with an outside diameter of 3 m and a length of 80 m and has a production capacity of 180 tons of CaO per day. Lime mud, with approximately 20 to 35% moisture content, is conveyed from the mud washers and fed to the kiln at the upper end thereof. As the mud moves through the kiln by gravity, it is dried and heated countercurrently by the combustion gases from an oil or gas burner at the lower end of the kiln. Calcination, which theoretically requires 428 kcal of heat per kg of CaCO.sub.3, begins at a location where the mud temperature reaches about 800.degree. C., in accordance with the equation: EQU CaCO.sub.3 (s).fwdarw.CaO(s)+CO.sub.2 (g) H=42.75 Kcal/mol
Lime mud calcining is a heat-transfer controlled process, i.e. the mud will decompose immediately if sufficient heat is supplied to raise its temperature above the calcination temperature, which varies from about 600.degree. to 870.degree. C., depending on the CO.sub.2 partial pressure of the surrounding gas. The higher the temperature, the more rapidly the heat is transferred to the material and the faster is the mud calcined. However, if the temperature is excessively high, in excess of 1200.degree. C., the resulting lime becomes "dead-burned" and less reactive. Maintaining the temperature between 900.degree. and 1100.degree. C. is critical in such prior art procedure for ensuring a high quality product.